The present invention relates to a CRT display device capable of suppressing unwanted external radiation of electromagnetic waves.
For a CRT display device like an electronic apparatus provided with a CRT (cathode ray tube), international standards stipulate upper limits of the intensity of electromagnetic waves radiated to the exterior from the CRT display device. One objective of these standards is to provide a specific level of safety which would prevent causing adverse effects on other nearby electronic apparatus. Thus, for marine electronics apparatuses, for example, of which many units are installed together with other units in a limited space, far more stringent standards are set forth to maintain navigation safety.
In a conventional CRT display device, a metallic shield cover used to be adopted to surround the outer periphery of a unit to suppress unwanted external radiation of electromagnetic waves. A CRT faceplate, however, cannot be covered with a shield cover made of a metal plate, so that a previous practice has been to cover the faceplate with a semitransparent filter covered with evaporated and deposited metal particles.
While a deflection yoke having horizontal deflection coils and vertical deflection coils, leads wires of the horizontal deflection coils and the vertical deflection coils, a high-voltage generating circuit formed by a flyback transformer, for instance, and its leads wires are sources of the aforementioned unwanted radiation, it is possible to effectively suppress the unwanted radiation of electromagnetic waves by making an enclosure of the CRT display device to serve as a shield cover. In addition, if the aforementioned semitransparent filter is provided on the CRT faceplate, unwanted electromagnetic radiation from the CRT faceplate can be suppressed to a certain degree. Unwanted electromagnetic emissions in a high-frequency range of tens to hundreds of megahertz (MHz), for example, can be sufficiently suppressed by this arrangement. However, it has scarcely been possible to suppress unwanted electromagnetic emissions at harmonic frequencies corresponding to several multiples of the horizontal deflection frequency, such as at tens to hundreds of kilohertz (kHz). Moreover, the use of aforementioned semitransparent filter could develop such problems that it would cause a reduction in screen brightness and a cost increase.
It is an object of the invention to provide a CRT display device effectively suppressing unwanted radiation of electromagnetic waves which could occur as a result of horizontal deflection or vertical deflection.
According to this invention, part of a deflection coil of a CRT or a lead wire of the deflection coil is used as a correction lead, and the correction lead is disposed inside an enclosure surrounding the CRT, wherein an electromagnetic field leaking from the CRT to the exterior is suppressed by an electromagnetic field generated by the correction lead. With this construction, an electromagnetic field produced in association with horizontal deflection or vertical deflection is canceled out by the electromagnetic field generated by the aforementioned correction lead and, in particular, electromagnetic fields leaking to the exterior through a faceplate of a CRT display device are suppressed in a reliable fashion.
According also to this invention, the correction lead is attached to a shield case surrounding the CRT at the rear of the CRT faceplate. Since the shield case surrounding the CRT has a fixed relative position relationship with a horizontal deflection coil or with a vertical deflection coil, the relationship between the phase and intensity of the electromagnetic field produced by the deflection coil and those of the electromagnetic field produced by the correction lead is kept unchanged. Furthermore, the invention facilitates positioning of the correction lead in the internal space of the enclosure. In addition, special components for arranging the deflection coil in the internal space of the CRT display device become unnecessary.
According also to this invention, a lead wire connecting a connector for the deflection coil that is connected to a connector on a circuit board on which a deflection drive circuit is provided and the deflection coil is used as the correction lead. With this construction, the correction lead can be provided simply by fitting a deflection yoke to the CRT, placing the lead wire of the deflection coil at specific positions, and connecting the connector at the end of the lead wire to the connector on the circuit board.
According also to this invention, the correction lead is fitted to specific points between the outer periphery of a CRT faceplate and the deflection coil. This construction serves to effectively suppress unwanted radiation from the deflection coil in the direction of the CRT faceplate.
According again to this invention, the correction lead connected to the horizontal deflection coil is placed at specific positions and thereby suppresses the fundamental component of a horizontal deflection frequency and its harmonic components radiated due to horizontal deflection of the CRT.